This invention relates generally to the production of shadow masks for color television tubes and particularly to an externally adjustable internal rotary seal and bearing structure for the fixed external and oscillating internal acid spray tubes associated with the etch tanks used to produce such shadow masks.
During the production of shadow masks for color television tubes, the shadow mask material is coated with a light sensitive acid resistant material. The coating is exposed to light to produce the aperture pattern and the profile of the shadow mask on the acid resistant material. After the exposure to light, the coating is developed and the exposed portions of the coating are washed away. Thereafter, the entire strip of shadow mask material is passed through an etchant or acid bath until the apertures are completely etched through and the profile of the shadow mask is partially etched through. The etching takes place in an etch tank which is inert to the corrosive action of the acid. However, the etching acid is provided to the etch tank from an external source through external tubes which communicate with internal tubes. The internal tubes have spray nozzles so that the acid sprays over the entire surface of the shadow mask strip. Uniform spraying is assured by oscillating the internal tubes about their longitudinal axis. Because the etching acid is provided from an external source, it is essential to effectly seal the tubes within the tank to prevent dangerous, damaging, and costly leakage of the acid outside the etch tank. It also is essential to provide a bearing and seal arrangement which permits relative oscillation between the external and internal tubes and which simultaneously seals to prevent leakage of acid. Such leakage can reduce the spray pressure of the etchant and thereby disturb the uniform flow of acid over the mask material resulting in nonuniform etching of the shadow masks.
In the prior art, two types of seals are presently available. One type is external to the etch tank, the other is internal to the etch tank. Both of these types of seals have inherent disadvantages. The external seal is disadvantageous because leakage occurs outside the tank and must be collected and disposed of. Additionally, the external seal is difficult to adjust to prevent leakage. It is customary to make seal adjustments while the process is operating in order to see the effect of the adjustment, and be certain that the seal is no tighter than necessary, for this would shorten the life of the seal. Therefore, the individual must make the adjustment in the proximity of the hot, leaking acid and thus, is in danger of being scalded and chemically harmed by exposure to the acid. Making the adjustment is also dangerous because the oscillating linkage is often in the proximity of the seal further endangering the mechanic. The internal seal is disadvantageous because leakage prevention adjustments must be made inside the tank, and thus cannot be made while the etching is in process. Because adjustments cannot be made while the system is operating, the minimum force required to prevent leakage cannot be determined. Force exceeding the minimum results in a greater torque required for oscillating the internal tube and the seal life is shortened.
The instant invention overcomes these disadvantages by the provision of a seal and bearing structure which is internally sealable and externally adjustable.